Recently, as NAND-type flash memories have become miniaturized, the reliability at a time of a write operation of data has been a problem. The NAND-type flash memory has memory cells connected in series, a source line connected to one end of the memory cells connected in series, and bit lines connected to the other end.
When data are written in this NAND-type flash memory, the applied voltage to the memory cells is changed in the following sequence. First, a write bypass voltage Vpass (for example, about 8 to 10 V), which is a level at which a threshold voltage is not changed, is applied to a control gate of memory cells that are not going to be written (i.e., nonselective memory cells), and the nonselective memory cells are set to an on state. Next, a program voltage Vpgm (for example, a voltage of 20 V or higher) is applied to a control gate of memory cells that are going to be written (i.e., selective memory cells), and electric charges are stored in a floating gate of selective memory cells. Therefore, the threshold voltage of the selective memory cells is raised (“0” data is written into the selective memory cells). The above describes the data write sequence.